The present invention relates to a connection device usable in surgical instrumentation of the spine. It relates more particularly to a device suitable for making the connection between screws for securing to a pedicle or to a joint, and a vertebral support rod that is to hold the vertebral column in a given alignment. The instrumentation comprising the screws, the connection device, and the vertebral support rod is used in particular for performing arthrodesis on a segment of the spine.
Document EP 0 425 783 discloses such a connection device constituted by a one-sided clamp for fixing a pedicle screw to a vertebral support rod having respective central axes situated in different planes. It is important to be able to vary the inclination of both axes, i.e. both the axis of the vertebral support rod and the axis of each pedicle screw, and,similarly it is important to be able to match the distance between the two axes to the shape desired by the surgeon.
In document EP 0 425 783, freedom in angular inclination is given by means of a ball constituting either the head of the pedicle screw or one end of the connection piece. The ball must necessarily be inserted in a socket and locked in position in the socket once the angular inclination has been set. When the ball constitutes the head of the pedicle screw, the socket forms a part of the connection piece, and in the embodiments described in that prior document, it is constituted by recesses formed in two distinct parts of the connection piece, which parts are hinged to each other like jaws, with the ball being locked by moving the jaws towards each other. Thus, when the pedicle screw is put into place, it must necessarily be passed beforehand through the part of the connection piece which acts as the bottom jaw, before it is itself inserted into the bone. The presence of the connection piece while the pedicle screw is being implanted can be an impediment for the surgeon.
In the embodiment where the ball constitutes one end of the connection piece, the socket is constituted by a recessed portion of the head of the pedicle screw. In that case, while the pedicle screw is being implanted, it is important for the head of the screw to be oriented in substantially the appropriate direction for passing the vertebral support rod, and that constitutes a constraint on the surgeon.
Elsewhere, document DE 195 12 709 describes an ostheosynthesis device comprising a screw for implanting, a connection piece, and fixing means for fixing the connection piece to the screw for implanting. The fixing means are constituted by two elements, each having a convex hemispherical portion. The screw for implanting has a concave hemispherical base and two threaded portions, one for implanting in the bone and the other for assembly with the connection piece. The connection piece is fixed to the screw for implanting by clamping the connection piece between the two elements forming the fixing means. The convex portion of each of the elements co-operates with a respective concave portion: one with the concave base of the head of the screw for implanting; and the other with a nut which has a concave portion and which is screwed onto the threaded portion for assembling the screw for implanting with the connection piece.
Unlike the device described in document EP 0 425 783, the device of DE 195 12 709 enables the connection piece to be released from the screw, thereby making the device easier to install.
Furthermore, since the elements forming the means for fixing the connection piece to the device comprise hemispherical portions with a common center, it is easy to vary the angular orientations of the axes of the screw and of the support rod without taking special precautions while implanting the screw in the spine in order to ensure that the screw is already initially oriented in substantially the desired direction.
Nevertheless, using such a device increases the amount of handling that the surgeon needs to perform. After implanting the screw in the spine, the surgeon must position the elements forming the above-described fixing means correctly, thereby increasing the length of time required to install the device. Furthermore, the presence of those fixing meats increases the""size of the connection device and that is not desirable for the patient.
The object the Applicant has set out to achieve is to propose a connection device which mitigates the drawbacks of the above-described devices.
It comprises an ostheosynthesis connection device for connecting a screw suitable for implanting in the spine to a vertebral support rod, the respective central axes thereof not lying in the same plane, the device comprising a connection piece, means for locking the vertebral support rod onto said connection piece, the screw for implanting being constituted by a bottom first threaded portion for implanting in the spine, a head having a curved base, and a top second threaded portion in line with the first threaded portion, the connection piece having, for connection with the screw, a hole of inside diameter greater than the outside diameter of the second threaded portion.
In characteristic manner, according to the invention, the head of the screw has flats, and the device also comprises a nut having a curved base and suitable for co-operating with the second threaded portion; the locking of the connection piece in position on the screw being obtained in characteristic manner, once the nut has been tightened, by the curved base of the nut bearing against the top bearing face of the connection piece, and by the bottom bearing face of the connection piece bearing against -the curved base of the head of the screw, the curved surfaces of the bases of the head of the screw, of the locking nut, and of the top and bottom bearing faces of the connection piece being portions of spheres all having their centers in the same position, one of the top and bottom bearing faces being convex, and the other being concave.
While the instrumentation is being put into place, the surgeon can implant all of the pedicle screws without constraint using a wrench on the flats, and without being concerned about the orientations of the screws. Each connection piece is put into place by inserting the top threaded rod through the hole having a curved seat, said threaded rod serving as a guide rod for putting said connection piece into place.
Since the top and bottom bearing surfaces of the connection piece are portions of concentric spheres, it is easy to adjust the angle between the screw and the vertebral support rod, and quasi-infinite angular variation in the respective positions of these two members is possible. Furthermore, the locking in position obtained in this way is particularly effective because the various members are held together, whatever their relative position, by pressing surface areas against one another.
In a preferred embodiment, the locking means for locking the connection piece on the vertebral support rod comprise:
a) a locking piece having a first passage for the vertebral support rod, a second passage for a portion of the connection piece, the first and second passages being substantially perpendicular and sharing a common inside volume in which the rod and the connection plate can come into contact, and a threaded third passage opening out into the inside volume of the first or second passage; and
b) a set screw suitable for screw engagement in the third passage to lock the vertebral support rod and the connection plate in position by clamping the set screw, the rod, the plate, and the piece against one another.
The presence of the locking piece sliding on the connection piece makes it possible to adjust the spacing between the screw for implanting and the vertebral support rod.
In this configuration, the connection piece does not have a slot as in document EP 0 425 783. The portion of the connection piece which is designed to slide in the second passage of the locking piece can have any configuration: preferably, it is a plate of substantially rectangular section with rounded sides, the second passage being shaped to enable fitted sliding of the locking piece over the portion of the connection piece that goes beyond the curved seat faces. The rectangular shape of the section of this portion of the connection piece also makes it possible to reduce the size of the locking piece in comparison to a portion having the same strength but being of circular section.
Similarly, this configuration makes it possible to fix the connection piece on the screw for implanting and then once that operation has been performed, to adjust the spacing between the vertebral support rod and the screw for implanting without acting on the way in which the connection piece is fixed to the screw and thus without any risk of disconnecting them or changing their respective positions.
In document DE 195 12 709, the spacing between the screw and the vertebral support rod is adjusted simultaneously while fixing the connection piece on the screw by positioning the second threaded rod in the hole in the connection piece which has a diameter greater than the diameter of the second threaded rod.
Preferably, the first passage for the vertebral support rod is of oblong cross-section, thereby allowing the rod to move a little in the height direction prior to the set screw being tightened and enabling an already curved rod to be passed therethrough.
In a preferred embodiment, the locking piece is a cylindrical piece, the first and second passages extending along diametral directions that-are perpendicular to each other, and the third passage being formed in one of the two circular faces of the cylinder and opening out into the inside volume of the first passage.
Advantageously, the connection piece has an abutment-forming element or is shaped in such a manner as to be capable itself of constituting an abutment for the locking piece while it is free to slide, prior to the set screw being tightened.
Advantageously, the top second threaded portion of the screw is suitable for being broken off, presenting a zone of weakness. In this particular disposition, after the curved-base nut has been tightened to lock the pedicular screw in position relative to the connection piece, it is possible to remove the top portion of the second threaded portion of the pedicular screw that projects from the nut, thereby minimizing the overall size of the instrumentation.